Please tell me you bought HREW tubing at least, not "pipe". Steel pipe isn't designed for what you're doing. I was also going to say that 1/4" is a little heavy, but still gusset it properly or it doesn't matter how thick the steel is.
And remember that your mounts need to go down the frame a decent distance to distribute the load, 1/2" plate won't help when it's only attached to 4" of frame.
Sorry if I sound like a dick, these are just things I've seen way to much in shop classes and the kids promptly rip the bumper off or bend the **** out of it while they're "testing". My buddies test procedure was to go to the junkyard and purposely run into a junk car and if the bumper didn't move it was good. Then again he sells custom bumpers now so he needed them to be beefy.